Thallus: crustose, verrucose-areolate, <3 mm thick; areoles: irregular, convex, widely attached or with constricted base, 0.5-2 mm in diam.; prothallus: indistinct surface: pale gray or greenish white to tawny yellow, smooth, esorediate medulla: white above, yellow in lower part, lacking calcium oxalate (H2SO4-) Apothecia: lecideine, immersed to broadly sessile, abundant, sometimes confluent, 0.2-1.8 mm in diam. disc: black, often pruinose, with whitish pruina consisting of calcium oxalate crystals, initially flat, soon convex or sinuous margin: black, distinct in young apothecia, later excluded proper exciple: 50-85 µm thick; lacking secondary metabolites; differentiated into a <20 µm wide, dark brown outer part with carbonized cells (< 5 µm; HNO3-, K-), and a pale central part, transient with the brown, <290 µm thick hypothecium (HNO3-) epihymenium: brown, pigmentation continuous with the outer exciple (HNO3-) hymenium: hyaline, inspersed with numerous oil droplets, 85-125 µm tall; tips of paraphyses: usually <3 µm wide with distinct apical caps asci: clavate, Bacidia-type, 80-96 x 17-26 µm, 8-spored ascospores: soon brown, submuriform to muriform, with <17 cells in optical section, ellipsoid, (18-)19.8-[21.9]-24(-31.5) x (10-)10.9-[12.1]-13.3(-16.5) µm, young spores with conspicuous lateral and septal thickenings, proper wall c. 0.4 µm thick, perispore c. 0.2 µm thick, ornamentation: relatively smooth Pycnidia: rather frequent, immersed but with upper part protruding, flask-shaped to globose and secondarily compartmentalized, wall pigmented all around but more strongly in upper part conidia: bacilliform, 4.5-5.5 x c. 1 µm Spot tests: thallus K+ yellow, C-, P+ yellow; medulla K+ red, C-, P- fluorescence: UV- iodine reaction: medulla not amyloid Secondary metabolites: diploicin (major), isofulgidin (minor), atranorin (minor), chloroatranorin (trace), dechlorodiploicin (trace), O-methyldiploicin (trace), an unknown diploicin derivative (minor), placodiolic acid (minor), cinnamomeic acid B (minor), and cinnamomeic acid C (trace). Substrate and ecology: epilithic, on acidic boulders, hard pebbles in conglomerate outcrops, and sandstone cliffs, on vertical or overhanging surfaces World distribution: Californian coast between Marin Co. and Santa Barbara Co. Sonoran distribution: southern California coast, including the Channel Islands (Anacapa, San Miguel, Santa Rosa, and Santa Cruz Islands), Baja California (Guadalupe Island). Notes: Buellia capitis-regum is characterized by a thick, verrucose-areolate thallus, a yellow medulla, and young ascospores with lateral and septal thickenings (Nordin 2000, Fig. 13A). The species cannot be confused with any other species, but it seems to be most closely related to the corticolous and lignicolous B. oidalea and B. oidaliella, both also restricted to the Pacific coast of North America. The cinnamomeic acids are responsible for the pigmentation of the lower part of the medulla.